They use tiny/huge numbers like 1e-10 or 1e10 to make approximations of the value. \frac{\text{d}}{\text{d}x} returns the derivative function, while \int returns the value of the definite integral. I'm also planning on writing an anti-derivative block as well.
You may want to use Simpson’s rule for (usually) much better results. If you do, you’ll beat the (authorized) SciSnap! library at numerical integration, hands down. 
Assuming you’re referring to a block reporting a function … you’re entering AI territory. Both interesting and challenging!
In the middle (𝛴) block, is "i" the same thing as "x"? 
I think I read about that in SICP, but I didn't really understand it. I'm open to learning, so I'll try to implement it!
Woops! I was learning about Riemann sums in class, and I guess I named the iterator wrong. I'll fix it...
